1. The Field of the Invention
The present invention relates to a zero insertion force connector for printed circuit boards or the like and in particular to a zero insertion force edge board connector utilizing a cam member and a contact drive member for sequentially bringing a plurality of contacts into and out of engagement with respective pads on a circuit board.
2. The Prior Art
The present invention relates to an improvement over the zero insertion force edge board connectors described in my U.S. Pat. Nos. 3,899,234 and 4,047,782. In order to differentiate between zero insertion force and low insertion force connectors, the term zero insertion force means there is no engagement of the connector contacts and the pads of the circuit board during insertion and extraction of the board. However, with low insertion force connectors, there may be some engagement between the connector terminals and the board pads, but it is not of sufficient magnitude to hinder the insertion and extraction movements. Zero insertion force connectors are preferable for those instances when it is desired to keep wear of the contacts and circuit board pads to a minimum.
There are many instances in the electronic industry when it is desirable and/or necessary to have sequencing of power, ground, and signals as they are applied to and removed from a circuit. There are both mechanical and electrical reasons for having such sequencing. It is always a fear of system designers that an electronic system could be subject to substantial damage through the inadvertent and unintentional removal of a component, such as a printed circuit board or lock card, from an energized system. This leads the circuit designer generally along one of three paths in being overly cautious in preventing such damage. First, the choice of logic may be limited because some types of MOS and other logics that are bi-polar in nature, with positive and negative power supplies referenced to ground, there is the possibility that if the ground is broken first, the supply voltages will "add" across an individual gate element, overheat it, and possibly destroy it. Secondly, he may be led to a jury rig system to prevent failure in this mode. This might include spring contacts in constant engagement with a metallic board or card guide which is connected to ground of the power supply so that it is absolutely the last thing to break before a board or card is fully disengaged from the system. However, metal card guides are quite expensive to design into a system. The third alternative would be to spend large amounts of money on precautions taken with the power supplies. This could include "crow bar" current sensing and "failure protect" schemes whereby the power supply is clamped to a zero output when the load is exceeded. The "crow bar" current sensing is a form of current limiting while the "fail-protect" is used for anti-overshoot and undershoot purposes during power up and power down conditions.
Digital systems are subject to fast moving transients. The data itself is indicated by change and thus is a transient. Removal of a board or a card from such a system, while under power, can generate a tremendous number of transient fake pulses which will simulate data on a bus and thereby cause other logic elements to produce errors. This could not happen if the power were removed to deactivate the card prior to removal of the card from the system. Furthermore, on each card there is often a series of capacitors called "decoupling units". These capacitors reduce minor fluctuations that occur on the card when possibility of all of the logic gates go from a 0 to a 1 state at the same time. This rapid change in load requirements could possibly lower the board voltage to below the logic margins and again cause errors. When a logic card is removed without sequentialling, it does just that. Power supplies that respond to these characteristics are quite expensive and highly sophisticated.